1. Field of the Invention
The present invention relates to a photovoltaic device having a highly reliable electrode structure and a process for producing said photovoltaic device. More particularly, the present invention relates to a photovoltaic device having a collecting electrode comprising a metallic wire and a bus bar and having an improved connection structure with respect to said metallic wire and bus bar.
2. Related Background Art
It is known to dispose a plurality of linear electrodes on the front surface of a solar cell in order to efficiently collect a electric power generated by the solar cell. It is also known to dispose a common bus bar at end portions of said linear electrodes in order to output the electric power collected by the linear electrodes to the outside.
Particularly, U.S. Pat. No. 4,260,429 discloses that as the collecting electrode disposed on the front surface of a solar cell, a plurality of metallic wires coated by a polymer material containing electrically conductive particles are fixed on the front surface of the solar cell by way of heat or/and pressure and a common bus bar is connected to end portions of the metallic wires.
U.S. Pat. No. 5,084,107 discloses a manner of providing a plurality of metallic wires having opposite end portions supported by clamps, applying an electrically conductive adhesive onto at least partial portions of the metallic wires, subjecting the metallic wires applied with the electrically conductive adhesive to heat treatment while pressing the metallic wires onto the front surface of a solar cell whereby solidifying the electrically conductive adhesive, and cutting the metallic wires, whereby collecting electrodes are formed on the front surface of the solar cell. This document also discloses that a collection electrode for collecting electric currents of the metallic wires is connected to the metallic wires.
Now, in order to connect the metallic wire as the collecting electrode with the bus bar, there are known a manner of using an electrically conductive paste, and another manner of using a solder. In the case of using the electrically conductive paste, there is often employed a manner of discharging the electrically conductive paste to predetermined several points using an applicator, followed by drying or curing the electrically conductive paste thus applied. In the case of using the solder, there is usually employed a manner of directly connecting the metallic wire with the bus bar by means of the solder.
However, any of these manners is liable to entail such problems as will be described below in the case of using such electrode configuration as described in the prior art in a solar cell.
(1) In the case of connecting a metallic member with the bus bar using the solder in order to integrate collecting electrodes, by-pass diodes, or photovoltaic elements (or solar cells) in series connection or parallel connection, the solder is fused. In this case, it is necessary to locally apply a high temperature heat of 250 to 350.degree. C., where the photovoltaic elements are liable to suffer from damages due to the heat applied thereon, resulting in deteriorating the characteristics of the photovoltaic elements.
(2) In the case of connecting the collecting electrode comprising the metallic wire with the bus bar by means of the solder, it is difficult to connect the collecting electrode with the bus bar through such adhesive layer of the foregoing electrically conductive adhesive. Therefore, it is necessary that the adhesive layer is partially removed and the soldering is conducted. In this case, bonding is necessary to be conducted at a number of points and because of this, the working efficiency is inferior.
(3) As the bus bar, a metal body having burs and which is liable to fracture is usually used. Therefore, the burs or fractured portions of the bus bar are liable to get into the semiconductor layer of the photovoltaic element whereby short-circuiting the photovoltaic element.
(4) In the case of connecting the metallic wire with the bus bar using an Ag-paste as the electrically conductive paste, it is necessary to use an appropriate applicator capable of discharging the Ag-paste in a desired amount and in addition to this, it is necessary to employ a plurality of steps of heating, drying, and curing the Ag-paste.
In this case, since the Ag-paste is applied on an uneven surface, it is difficult to attain a uniform thickness for the resulting Ag-paste layer. This situation is liable to entail a problem in that it is difficult to attain a resin sealing with no defect such as air bubble for the photovoltaic element in the lamination process for sealing a photovoltaic device comprising the photovoltaic element.
(5) In the case of using the solder, when a flux is together used or a resin flux cored solder is used in order to attain good soldering, it is necessary to separately conduct the steps of removing the flux or the resin flux by way of ultrasonic cleaning or the like and drying the resultant.
(6) In any of the above described manners, when the metallic wire as the collecting electrode is extended and arranged on the front surface of the photovoltaic element, it is provisionally fixed to a non-power generation region of the photovoltaic element. In this case, there is a problem in that it is necessary to have an extra area for the provisional fixing other than the bus bar-forming area.